/Engine/Source/Runtime/Engine/Classes/Engine/TextureRenderTarget2D.h
Calculates the size of this texture if it had MipCount miplevels streamed in. This is the size in GPU memory of the built platformdata, accounting for LODBias, etc.
Total size of all specified mips, in bytes. Returns 0 for error.
virtual uint32 CalcTextureMemorySizeEnum ( ETextureMipCount Enum ) const